A dangerous heat wave expected to break records has settled over much of the West Coast, with over 150 million Americans under excessive heat watch and heat advisory alerts on Friday, forecasters said.

The heat is expected to become more widespread in the West on Friday and into Saturday, with no relief expected for days. On Friday, temperatures are set to be 15 to 30 degrees higher than normal throughout the region, but Saturday is anticipated to be the hottest day with temperatures that will match or break records.

Much of California and southern Oregon will see temperatures in the 100s and 110s Friday. On Saturday, California should expect 110s away from the oceans and higher elevations, while parts of the Desert Southwest could get into the 120s. Officials in Oregon's Multnomah County in Oregon declared a state of emergency beginning Friday through Monday due to the heat.

"The duration of this heat is also concerning as scorching above average temperatures are forecast to linger into next week," the National Weather Service said.

Meanwhile, heat and humidity were making their way east on Friday, with above-average temperatures kicking off the weekend.

Extreme heat increases wildfire risk

With the extended heat comes elevated risk for any flame to become a fast-growing wildfire, as California already battles flames that sprang up in recent days. One fire, the French Fire, started on Thursday and quickly burned hundreds of acres, forcing evacuations in Mariposa County.

More than 5 million Americans on Friday were under fire weather watches or red flag warnings, which mean elevated temperatures, low humidity and winds that combine to pose a risk of wildfire.

The Los Angeles area is experiencing elevated to critical fire conditions through at least Saturday morning in its interior areas, extending to southern Santa Barbara County and the Ventura and Santa Barbara County mountains, the weather service there said.

"Use extra caution with any source of flame! Any new grass fire will grow rapidly, and could spread into brush," the weather service in Los Angeles said.

'Oppressive' heat and humidity shifts east this weekend

It's not just the West feeling the effects of extreme heat. The weather service said "oppressive" heat and humidity will also blanket blanket the Mid-Atlantic and Southeast with high temperatures in the upper 90s and low 100s to start the weekend.

The heat index could soar into the 110s in parts of the Mississippi Valley, the National Weather Service said. Overnight temperatures in the 70s and 80s won't offer much relief, either.

Where have high temps broken records?

The heat has already broken or matched records in the West in recent days, and the worst is yet to come.

• Palmdale Regional Airport in California recorded a high of 110 degrees on Thursday, tying the record set in 1973.
• In San Francisco, the regional airport saw a high of 87 degrees on July 4, breaking the previous record of 85 in 1973.
• Phoenix's overnight temperature broke a record warm low temperature, as recorded at Sky Harbor airport. The low was 93 in the early morning hours Friday, breaking the previous record of 91 degrees set in 1996.
• On July 3, the city of Livermore, California, in Alameda County hit 110 degrees, breaking the record of 109 for that day in 2001.